1. Field of the Invention
This invention pertains to the field of producing substantially carbon-free silicon for semiconductor use. More specifically, the present invention relates to a graphite component, such as a crucible, having an outer layer of glassy carbon formed from a thermoset organic resin and in which the component is used in the process for producing silicon crystal growth from a molten silicon bath containing dopants. The invention also relates to a method for producing the coated graphite component.
2. Background of the Invention
Generally, polycrystalline silicon rods are made by the pyrolytic decomposition of a gaseous silicon compound, such as silane or a chlorosilane (e.g., trichlorosilane) on a rod-shaped, red-heated starter filament made preferably from a silicon seed rod or, alternatively, from a high-melting point metal having good electrical conductivity such as tungsten or tantalum. The principles of the design of present state-of-the-art reactors for the pyrolysis of silane and chlorosilanes are set forth in, for example, U.S. Pat. Nos. 3,147,141; 4,147,814; and 4,150,168, which are incorporated herein by reference as if set out in full. It is generally more desirable to prepare the polycrystalline silicon rods by silane pyrolysis so as to avoid the complications caused by the formation of chloride by-products when pyrolyzing chlorosilanes.
The pyrolysis of silane to form silicon and hydrogen, or a chlorosilane which produces chloride-containing compounds such as HCl, SiHCl.sub.2 or the like as well as hydrogen, is performed in a reactor consisting of a series of heated filaments, generally silicon rods, surrounded by cooled surfaces. Typically, the filaments are heated by introducing an electrical current through the filament. The process is started with the silicon filament at ambient temperature.
The polycrystalline silicon is produced by heterogeneous decomposition of the silane, chlorosilane or mixtures thereof on the glowing hot silicon filament rod. The reaction deposits silicon on the surface of the rod and releases hydrogen gas if the silicon is formed by the decomposition of silane, or hydrogen gas in conjunction with other chloride-containing by-product compounds if the source of silicon is chlorosilane.
One of the major objectives in the production of polycrystalline silicon is to produce a silicon rod which is as pure as possible. Even slight amounts of contaminants have a major impact on the efficacy of the silicon chips which are ultimately made from this precursor polycrystalline silicon. The prior art techniques for making polycrystalline silicon have had the problem of coping with various contaminants, including carbon.
As stated above, the gaseous silicon compound which is used as a source for the silicon, is thermally decomposed by means of a heated starter filament. This filament, typically made of a silicon seed rod, is generally heated by passing an electrical current therethrough. Accordingly, this filament must be held securely in place so as to accommodate the growing polysilicon rod that is being deposited thereon while simultaneously being capable of having an electrical current passed through it. Typically, a graphite chuck has been utilized by the prior art in order to accomplish both of these objectives. The graphite chuck is made so that the starter filament may be securely mounted on it. The chuck can be positioned and seated on an electrode which provides the necessary electrical power for the required current, and, most importantly, the chuck is electrically conductive so as to be able to conduct the current from the electrode to the filament.
Polycrystalline rods are primarily used as precursors for making silicon crystal growth for the semiconductor industry by either the float zone melting process or by the Czochralski crystal pulling technique. These silicon crystal growths or rods are then processed to form silicon wafers from which silicon chips are made for use in the electronic industry.
In the Czochralski process for making silicon crystals, chunks of polycrystalline silicon rods and dopant are melted in a silica lined graphite crucible. Dopants are added to the silicon bath to create the required electrical characteristics and the silicon structure is rearranged to a single crystal form. Heating, such as radio frequency heating, is required to achieve a 1,425.degree. C. melting temperature for the silicon rods. A seed crystal of either [111] or [100] orientation contacts the molten silicon bath and is then slowly raised. The surface tension between the seed and the molten silicon bath causes a small amount of the liquid to rise with the seed. Upon cooling, the atoms in the melt orient themselves to the structure of the seed, repeating the seed orientation in the growing crystal. To achieve doping uniformity and diameter control, the seed and crucible are rotated in opposite directions. In this method of producing crystals four feet long or longer and six inches or more in diameter, it is important to insure that the component parts employed in the formation of the silicon crystal growth do not contaminate the silicon product. Graphite parts, including crucibles, pedestals (shafts) and heaters, are generally used in the Czochralski crystal growth process. It is therefore necessary to protect the silicon crystals from contamination of carbon from these graphite parts. Even though the graphite component is not in direct contact with the molten silicon, there is potential for carbon contamination of the silicon crystals. For example, carbon from the graphite crucible could diffuse into the silica liner which could then be diffused into the silicon bath. Silicon containing vapors formed during the process can also react with the graphite component generating carbon contaminants.
It is an object of the present invention to provide a graphite component, such as a crucible, coated with a glassy carbon layer that will prevent the reaction of the carbon in the component from contaminating the molten silicon bath which could result in impure silicon wafers.
It is another object of the present invention to provide a method for producing a glassy carbon coated graphite component that can be used in the production of substantially carbon-free silicon wafers which in turn can be used to make high quality silicon chips.
The foregoing and additional objects will become more fully apparent from the following description.